The present invention relates to a measuring head for the inductive measurement of the thickness of an insulating layer of an electric conductor, of the type which comprises a coil and a contact part designed to be placed on the conductor having the insulating layer to be measured.
A measuring head of this kind for measuring the thickness of the insulation on cables is disclosed in DE-AS No. 1,673,888. The contact surface of the head has a groove for receiving the cable to be measured. Other measuring heads use a flat contact surface. A measuring head having a groove for a cable can only be used for a particular range of diameters, so that at least the part of the measuring head containing the contact surface must be changed for diameters outside this range. Further, both the measuring heads with a groove and the measuring heads with a flat contact surface have one serious disadvantage, namely unacceptably high errors in measurement which occur when the surface of the conductor does not have a geometrically simple form such as a flat or a cylindrical surface.
Cable sheaths are always more or less irregular in structure and are therefore not perfectly circular. As a result, the measuring head will from time to time come to lie at a position on the cable which is situated at some distance from the point to be measured even if the cable is very accurately guided and positioned. The wall thickness recorded in the measurement will then deviate from the true wall thickness and a false result will be obtained.
There have already been attempts to use moving measuring heads which adapt to the surface of the cable, but these have been found to be highly susceptible to trouble and have therefore not become established in practice.
The object of the present invention is to provide a measuring head which is so designed that the irregular structure of a cable sheath cannot falsify the measurement and errors in measurement due to imperfect contact of the measuring head with the cable sheath are eliminated.